If you have started poking around the coding in Pokémon Go, you would have found evidence for the existence of the rarest first-generation Pokémon that is expected to soon make its present in the mobile game.
Are you trying to catch them all in the Pokémon Go game? If so, you might be frustrated with the rarity of some characters. Mew is the 151st Pokémon that is originally obtainable only by means of special download events for the games for Game Boy. This is definitely one of the rarest Pokémon characters.
Mew is referred to as Myhtic and its elemental type is Psychic. It seems to have some highest stats in the game. Unlike the other Pokémon characters, you will not come across Mew or Mewtwo that have a value for the base capture rate as neither are catchable in the wild.
A Reddit user "_istin_" looked at the complete coding behind Niantic's game. The others have highlighted information that was revealed by the data-mine and this includes references to Mew as well.